The search for new physics is one of the key goals of the ATLAS collaboration. With the discovery of the Higgs boson in 2012, the Standard Model gained an essential ingredient for understanding fundamental particles and their interactions – but it cannot yet be considered complete. The nature of dark matter, which makes up about 27% of the Universe, remains a major open question. Dark matter could be part of a complex “dark sector” of particles beyond the Standard Model, with its own structure of internal symmetry and interactions. Among these “dark particles”, the “dark photon” is a predicted mediating particle for interactions in this new sector. If dark photons interact with Standard Model particles, they could be produced in high-energy proton-proton collisions at the LHC and detected by the ATLAS experiment.
The ATLAS collaboration searched for signs of dark photons in data collected by the experiment during LHC Run 2 (2015-2018). Their newest search targets, for the first time in ATLAS, the production of a Higgs boson in association with a Z boson, with the subsequent decay of the Higgs into a photon and a dark photon.
A dark photon would leave no visible signal in the ATLAS detector because it is predicted to interact extremely weakly with Standard Model particles. However, it would manifest itself as an imbalance in the total energy in the transverse plane of the detector ( lack of transverse energy). For their new search, the physicists selected collision events with two leptons (originating from the decay of the Z boson), one photon, and no transverse energy. Multiple Standard Model processes can give rise to a similar signature; they were modeled as accurately as possible using simulations and data-driven estimations.
In addition, the researchers used a machine learning technique (Boosted Decision Tree, BDT) to discriminate between events that are more likely to be from a dark photon (classified with a high BDT score) from those coming from standard model processes (classified with a lower BDT score). . If a dark photon were produced, it would be observed as an excess of high BDT score events over those expected from Standard Model processes. Physicists have searched for dark photons with a wide range of possible masses, ranging from massless to 40 GeV. No excess was observed. This allowed the researchers to establish exclusion limits on the rate at which the Higgs boson decays into a photon and a dark photon: the results show that the maximum allowed rate varies from 2.3% for a massless dark photon to 2, 5% for the other photon considered. masses.
The ATLAS collaboration has established the best exclusion limits at the LHC for the search for Higgs bosons that decay into a photon and dark photon.
Similar results were obtained by a previously published analysis, looking for the same Higgs boson decay, but in a different Higgs production mode (Vector Boson Fusion, characterized by a larger cross section). This second analysis ruled out the decay of the Higgs boson to massless dark photons at rates greater than 1.8%. The data were also used to place strong constraints on scenarios with additional Higgs-like bosons decaying in the same way.
In conclusion, the ATLAS Collaboration has established the best exclusion limits at the LHC in the search for Higgs bosons that decay into a photon and dark photon. These remarkable results were made possible by the excellent performance of the ATLAS experiment during Run 2 of the LHC and by the highly sophisticated and precise data analysis techniques developed by ATLAS members. The results, together with those from other accelerators or astrophysics experiments, are of great interest to theorists to validate their models and improve their predictions. ATLAS researchers are looking forward to the larger data set expected from LHC Run 3, which will allow them to make more precise measurements.